Efficient Composite Amplifier

ABSTRACT

A detuned composite amplifier includes a nonlinear drive function ( 22 ) that has a phase that varies with the composite amplifier output voltage amplitude. The nonlinear drive function ( 22 ) is configured to transform the output voltage transition point of the prior art into an extended output voltage transition region to increase the efficiency of the composite amplifier.

TECHNICAL FIELD

The present invention relates to composite amplifiers, and especially toa method and arrangement for increasing the efficiency of suchamplifiers.

BACKGROUND

In many wireless communications systems, the power amplifier (PA) in thetransmitter is required to be very linear, in addition to being able tosimultaneously amplify many radio channels (frequencies) spread across afairly wide bandwidth. It also has to do this efficiently, in order toreduce power consumption and need for cooling, and to increase itslongevity. High linearity is required, since nonlinear amplifiers wouldcause leakage of interfering signal energy between channels.

The amplitude probability density of a mix of sufficiently manyindependent radio frequency (RF) channels, or of a multi-user CDMA (CodeDivision Multiple Access) signal, tends to be close to a Rayleighdistribution having a large peak-to-average power ratio. Since aconventional RF power amplifier generally has an efficiency proportionalto its output amplitude, its average efficiency is very low for suchsignals.

In response to the low efficiency of conventional linear poweramplifiers, many methods have been proposed. Two of the most promisingare the Chireix outphasing method [1], and the Doherty method [2].

To minimize the costs for producing the efficient power amplifiersdescribed in the previous sections, one would like to avoid trimming.Since component values and electrical lengths of transmission lines varybetween the produced amplifiers, they will all be more or less detunedor off balance. One problem that so far has remained unsolved is how toobtain maximum efficiency (i.e. best possible under such conditions)from an imperfect composite amplifier.

SUMMARY

An object of the present invention is to efficiently drive a detunedcomposite power amplifier.

This object is achieved in accordance with the attached claims.

Briefly, the present invention provides a nonlinear drive function thathas a phase that varies with the composite amplifier output voltageamplitude. This feature enables splitting of the described transitionpoints into transition regions, which increases amplifier efficiency. Infact this feature may actually be used to deliberately detune amplifiersto increase efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a block diagram of a Chireix amplifier;

FIG. 2 is a block diagram of a Chireix amplifier with a modified outputnetwork;

FIG. 3 is a diagram illustrating how the amplitude of the output voltageof the constituent amplifiers depends on the normalized Chireixamplifier output voltage;

FIG. 4 is a diagram illustrating how the phase of the output voltage ofthe constituent amplifiers depends on the normalized Chireix amplifieroutput voltage;

FIG. 5 is a diagram illustrating how the amplitude of the output currentof the constituent amplifiers depends on the normalized Chireixamplifier output voltage;

FIG. 6 is a diagram illustrating how the phase of the output current ofthe constituent amplifiers depends on the normalized Chireix amplifieroutput voltage;

FIG. 7 is a diagram of the nonlinear function of a Chireix amplifier;

FIG. 8 is a diagram illustrating how the efficiency of the Chireixamplifier depends on the normalized Chireix amplifier output voltage;

FIG. 9 is a block diagram of a Doherty amplifier;

FIG. 10 is a diagram illustrating how the amplitude of the outputvoltage of the main and auxiliary amplifier, respectively, depends onthe normalized Doherty amplifier output voltage;

FIG. 11 is a diagram illustrating how the phase of the output voltage ofthe main and auxiliary amplifier, respectively, depends on thenormalized Doherty amplifier output voltage;

FIG. 12 is a diagram illustrating how the amplitude of the outputcurrent of the main and auxiliary amplifier, respectively, depends onthe normalized Doherty amplifier output voltage;

FIG. 13 is a diagram illustrating how the phase of the output current ofthe main and auxiliary amplifier, respectively, depends on thenormalized Doherty amplifier output voltage;

FIG. 14 is a diagram of the nonlinear function of a Doherty amplifier;

FIG. 15 is a diagram illustrating how the efficiency of the Dohertyamplifier depends on the normalized output voltage;

FIG. 16 is a block diagram of a detuned Chireix amplifier;

FIG. 17 is a diagram illustrating how the amplitude of the outputvoltage of the constituent amplifiers in FIG. 16 depends on thenormalized Chireix amplifier output voltage;

FIG. 18 is a diagram illustrating how the phase of the output voltage ofthe constituent amplifiers in FIG. 16 depends on the normalized Chireixamplifier output voltage;

FIG. 19 is a diagram illustrating how the amplitude of the outputcurrent of the constituent amplifiers in FIG. 16 depends on thenormalized Chireix amplifier output voltage;

FIG. 20 is a diagram illustrating how the phase of the output current ofthe constituent amplifiers in FIG. 16 depends on the normalized Chireixamplifier output voltage;

FIG. 21 is a diagram of the nonlinear function of the Chireix amplifierin FIG. 16;

FIG. 22 is a diagram illustrating how the efficiency of the Chireixamplifier in FIG. 16 depends on the normalized Chireix amplifier outputvoltage;

FIG. 23 is a block diagram of a detuned Doherty amplifier;

FIG. 24 is a diagram illustrating how the amplitude of the outputvoltage of the main and auxiliary amplifier, respectively, in FIG. 23depends on the normalized Doherty amplifier output voltage;

FIG. 25 is a diagram illustrating how the phase of the output voltage ofthe main and auxiliary amplifier, respectively, in FIG. 23 depends onthe normalized Doherty amplifier output voltage;

FIG. 26 is a diagram illustrating how the amplitude of the outputcurrent of the main and auxiliary amplifier, respectively, in FIG. 23depends on the normalized Doherty amplifier output voltage;

FIG. 27 is a diagram illustrating how the phase of the output current ofthe main and auxiliary amplifier, respectively, in FIG. 23 depends onthe normalized Doherty amplifier output voltage;

FIG. 28 is a diagram of the nonlinear function of the Doherty amplifierin FIG. 23;

FIG. 29 is a diagram illustrating how the efficiency of the Dohertyamplifier in FIG. 23 depends on the normalized output voltage;

FIG. 30 is a block diagram of a Chireix amplifier with a currentlimitation at low output amplitudes;

FIG. 31 is a diagram illustrating how the amplitude of the outputvoltage of the constituent amplifiers in FIG. 30 depends on thenormalized Chireix amplifier output voltage;

FIG. 32 is a diagram illustrating how the phase of the output voltage ofthe constituent amplifiers in FIG. 30 depends on the normalized Chireixamplifier output voltage;

FIG. 33 is a diagram illustrating how the amplitude of the outputcurrent of the constituent amplifiers in FIG. 30 depends on thenormalized Chireix amplifier output voltage;

FIG. 34 is a diagram illustrating how the phase of the output current ofthe constituent amplifiers in FIG. 30 depends on the normalized Chireixamplifier output voltage;

FIG. 35 is a diagram of the nonlinear function of the Chireix amplifierin FIG. 30;

FIG. 36 is a diagram illustrating how the efficiency of the Chireixamplifier in FIG. 30 depends on the normalized Chireix amplifier outputvoltage;

FIG. 37 is a block diagram of a Chireix amplifier with a currentlimitation at high output amplitudes;

FIG. 38 is a diagram illustrating how the amplitude of the outputvoltage of the constituent amplifiers in FIG. 37 depends on thenormalized Chireix amplifier output voltage;

FIG. 39 is a diagram illustrating how the phase of the output voltage ofthe constituent amplifiers in FIG. 37 depends on the normalized Chireixamplifier output voltage;

FIG. 40 is a diagram illustrating how the amplitude of the outputcurrent of the constituent amplifiers in FIG. 37 depends on thenormalized Chireix amplifier output voltage;

FIG. 41 is a diagram illustrating how the phase of the output current ofthe constituent amplifiers in FIG. 37 depends on the normalized Chireixamplifier output voltage;

FIG. 42 is a diagram of the nonlinear function of the Chireix amplifierin FIG. 37;

FIG. 43 is a diagram illustrating how the efficiency of the Chireixamplifier in FIG. 37 depends on the normalized Chireix amplifier outputvoltage;

FIG. 44 is a block diagram of a detuned Chireix-Doherty amplifier;

FIG. 45 is a diagram illustrating how the amplitude of the outputvoltage of the constituent amplifiers in FIG. 44 depends on thenormalized Chireix-Doherty amplifier output voltage;

FIG. 46 is a diagram illustrating how the phase of the output voltage ofthe constituent amplifiers in FIG. 44 depends on the normalizedChireix-Doherty amplifier output voltage;

FIG. 47 is a diagram illustrating how the amplitude of the outputcurrent of the constituent amplifiers in FIG. 44 depends on thenormalized Chireix-Doherty amplifier output voltage;

FIG. 48 is a diagram illustrating how the phase of the output current ofthe constituent amplifiers in FIG. 44 depends on the normalizedChireix-Doherty amplifier output voltage; and

FIG. 49 is a diagram illustrating how the efficiency of theChireix-Doherty amplifier in FIG. 44 depends on the normalizedChireix-Doherty amplifier output voltage.

DETAILED DESCRIPTION

In the following description the same reference designations will beused for the same or similar elements throughout the figures of thedrawings.

Furthermore, although they are not identical, the output networks ofboth Chireix and LINC amplifiers will be denoted Chireix type outputnetwork or combiner.

Before the invention is described in detail, the Chireix and Dohertyamplifier and known variations thereof will be briefly described.

FIG. 1 is a block diagram of a typical prior art Chireix amplifier. Theterm “outphasing”, which is the key method in Chireix and LINCamplifiers, generally means the method of obtaining amplitude modulationby combining two phase-modulated constant-amplitude signals produced ina signal component separator 10. After up-conversion and amplificationthrough RF chains 12, 14 (mixers, filters, amplifiers) and poweramplifiers PA1, PA2, the outphased signals are combined to form anamplified linear signal in a Chireix type output network 20. The phasesof these constant-amplitude outphased signals are chosen so that theresult from their vector-summation yields the desired amplitude. Outputnetwork 20 includes two quarter-wave lines λ/4 (where λ is thewavelength corresponding to the center frequency of the amplifier) andtwo compensating reactances +jX and −jX, which are used to extend theregion of high efficiency to include lower output power levels. In [3,4] the efficiency of Chireix systems is analyzed. In [5, 8, 9] methodsto overcome nonlinearity due to gain and phase imbalances are described.The Chireix method has also been used in broadcast transmitters underthe trademark Ampliphase [6, 7]. According to an enhancement describedin [21, 13] the constituent amplifiers should be driven linearly belowthe transition point to increase the efficiency.

An advantage of the Chireix amplifier is the ability to change theefficiency curve to suit different peak-to-average power ratios, bychanging the size (X) of the reactances. The peak output power isequally divided between the amplifiers irrespective of this adjustment,which means that equal size (equal maximum output power) amplifiers canbe used.

Another embodiment of the output network of a Chireix amplifier can bebuilt by shortening and lengthening the λ/4-lines by a quantity δ, whilekeeping the sum of the two lines at λ/2, instead of using compensatingreactances. Such an embodiment is described in [16] and illustrated inFIG. 2.

The amplitudes and phases of the constituent amplifier (PA1 and PA2)output node voltages and output currents for a Chireix amplifier withlengthened and shortened transmission lines in the output networkaccording to [16] are shown in FIG. 3-6. The transition point T is hereat 0.4 of the maximum output voltage, which is optimal for thisamplifier.

According to [16] the Chireix amplifier drive signals from signalcomponent separator 10 in FIG. 2 include a linear plus or minus anonlinear component (plus to one power amplifier and minus to the otherpower amplifier). By separating each constant-amplitude phase-modulatedsignal from a standard signal component separator 10 into a linear partand a modified nonlinear part, changing the amplitude and phase of thesecomponents individually according to a set of specific rules, andrecombining the parts into a signal with new properties according to[16], it is possible to obtain a Chireix amplifier with substantiallylower drive power consumption than the standard Chireix amplifier. Thenonlinear function of the input signal for the drive decomposition of[16] is shown in FIG. 7. It is important to note that even though thevoltages and currents in the Chireix amplifier have varying phases, thedecomposed nonlinear function has the same phase at all amplitudes. Thisnonlinear function is applied to the output currents of both constituentamplifiers.

For the Chireix amplifier illustrated in FIG. 2, built with transmissionlines of 0.32λ and 0.18λ electrical length according to [16], theefficiency curve peaks at 0.47 of the maximum output voltage, as shownin FIG. 8.

FIG. 9 is a block diagram of a typical prior art Doherty amplifier. TheDoherty amplifier uses one linear and one nonlinear power amplifierbranch. The published theory states that a main power amplifier PA1 isdriven as a linear amplifier in class B, and an auxiliary poweramplifier PA2 having nonlinear output current (through class C operationor some other technique represented by block 22) “modulates” theimpedance seen by the main amplifier, through the impedance-invertingquarter-wave line (see [2, 10]) in the output network. Since thenonlinear output current of the auxiliary amplifier is zero below acertain transition (output) voltage, the auxiliary amplifier does notcontribute to the power loss below this voltage. An alternative outputstructure with built-in impedance matching is described in [11, 12].

The transition point T of the standard Doherty amplifier is at half themaximum output voltage. With this transition point the efficiency curveis most suited for moderate peak-to-average power ratios, and the peakpower is divided equally between the two constituent amplifiers. Thetransition point in the Doherty amplifier can be changed by changing theimpedance of the quarter-wave transmission line (or equivalent circuit).The efficiency curve can then be adjusted for higher peak-to-averagepower ratios, and the peak output power will be unequally dividedbetween the amplifiers. Different size amplifiers will thus be neededfor optimum utilization of the available peak power.

The amplitudes and phases of the constituent amplifier (i.e. transistor)output node voltages and output currents versus the normalized outputvoltage, for a Doherty amplifier with two equal amplifiers, are shown inFIG. 10-13.

The nonlinear function 22 used in the original Doherty amplifier has theshape of the auxiliary amplifier (PA2) output current. The nonlinearfunction of the input signal according to the unified drivedecomposition of [16] is shown in FIG. 14. This nonlinear function hasthe same phase at all amplitudes and is optimal for a perfectly tunedDoherty amplifier. It is included here as a reference for the nonlinearfunctions of the present invention described below.

For the Doherty amplifier the efficiency curve has a peak at thetransition point, which for the exemplary amplifier with equalconstituent amplifiers is at 0.5 of the maximum output voltage, as shownin FIG. 15.

Recently a new theory for the Doherty amplifier has been developed, thatled to the invention of wideband distortion-canceling andefficiency-optimized Doherty amplifiers [14]. Thereafter, a new way ofoperating Chireix amplifiers with increased efficiency was devised [15],which at the same time allows the distortion-canceling method of theinvention in [14] to be utilized.

A unified way to build and operate (while having linear output) bothDoherty and Chireix amplifiers, as well as most others, is described in[16].

Well-designed amplifiers with three or more independently controlledtransistors can be used to increase efficiency above what was previouslyattainable with multistage Doherty amplifiers [10] having the samenumber of transistors. Combinations of Chireix and Doherty properties atdifferent output levels have been described in [17] (multistage Chireixstructures and drive), [18] (combinations of Doherty and Chireixstructures and drive), and [19] (improved drive for a prior artamplifier structure).

A drawback of all the described systems is that the described methodsfor driving them are maximally efficient only for perfectly tunedsystems. Thus, if the power amplifiers or the output networks aremismatched, the efficiency is decreased. The present invention describesa method for increasing efficiency also for detuned compositeamplifiers.

The solution involves “splitting” a region between two transition points(or a transition point and an end point) in a composite amplifier intotwo new regions. In some amplifiers this is easier understood assplitting the single transition point into two transition points with anew intermediate region inbetween. Efficiency can be increased comparedto the previous solutions irrespective of the amount of detuning orvariations in component values. The problem of obtaining maximumefficiency from an imperfect amplifier is solved by this method.

Both state of the art solutions, the Doherty amplifier [2] and themodified Chireix amplifier [21, 13], use a single transition point T. Inthe Doherty case, the main transistor works alone at low output levels,and the auxiliary transistor is used only above the transition point,see FIG. 12. In the modified Chireix amplifier, the amplifier is drivenin “outphasing” mode above the transition point, and both constituentamplifiers (transistors) are driven linearly with equal amplitude andconstant phase difference below this point.

It has been shown [16] that the output network drive (“output”,“excitation”) currents can be generally decomposed into linear parts andnonlinear parts, where the linear parts provide the actual outputsignal, and the nonlinear parts, which are forced not to be seen in theoutput signal by having a special relationship between them [14], areused to reduce the average output current amplitudes. The solutionspresented here do not change any of this, but only modify the shape ofthe nonlinear drive current parts to more optimally reduce the averagecurrents. The modification allows the nonlinear drive current part tohave different phases at different amplitudes.

How the solution in accordance with the present invention is implementedin different amplifiers will now be examined in a number of differentcases. In all examples (including the previous ones) the maximum outputvoltage and also the maximum node voltages are normalized. The outputnetwork structure from [16], shown in FIG. 2, is used and the loadresistance is set to one. This means that the sum of the inverses of theoptimum load resistances for the transistors is also equal to one, as isthe sum of the maximum output RF currents. The first two examples areimprovements over the state of the art solutions. Examples 3 and 4describe how the solution can be used to improve the system in specialcases. Example 5 describes a three-transistor amplifier that combinesproperties from the first two examples. The generic structure of FIG. 2is used also for example 5, with an extra branch consisting of atransistor PA3 and a transmission line connecting it to the commonoutput (where the transmission lines from PA1 and PA2 join). In theseexamples RP chains 14, 16 will be omitted to avoid cluttering of thedrawings, since they are not necessary to explain the present invention.

EXAMPLE 1 Detuned Chireix Amplifier

By letting the sum of the transmission line lengths differ slightly from½, we have a “detuned” Chireix amplifier. In the case presented here,the electrical lengths are 0.30λ (from PA1 to the common output) and0.18λ (from PA2) which sums to 0.48λ (instead of the correct value0.5λ), as illustrated in FIG. 16. This can be seen as a Chireixamplifier operating at 4% below its nominal frequency. On the inputside, linear drive signal components are produced by amplifier/phaseshifter 26 and 32 directly from the input signal. Similarly, nonlineardrive signal components are generated by a nonlinear element 38 andamplifier/phase shifter 28 and 30. Unit 38 may, for example, beimplemented as a combination of a lookup table followed by D/Aconverters in which a digital input signal amplitude is transformed intothe proper drive signals (the input signal amplitude is assumed to beproportional to the composite amplifier output voltage amplitude).However, analog implementations are also possible. The nonlinear signalfrom amplifier/phase shifter 28 is added to the linear signal componentfrom amplifier/phase shifter 26 in an adder 34, while the nonlinearsignal from amplifier/phase shifter 30 is subtracted from the linearsignal component from amplifier/phase shifter 32 in an adder 36. In ananalog embodiment adders 34, 36 may, for example, be realized ashybrids. In a digital embodiment they are digital adders. The amplitudesand phases of the output node voltages and output currents for thisamplifier are shown in FIG. 17-20.

Comparing with the Chireix amplifier behavior of FIG. 3, thecharacteristics of the new solution are shown in the voltage diagram asa faster rise in amplitude at the PA1 output node than at PA2 at lowoutputs. Above that, a region follows where PA1, but not PA2, is atconstant voltage. An upper region of both amplifiers being at constantvoltage is then used. The difference between this kind of operation andthe state of the art operation of Chireix amplifiers [13], where bothamplifiers have the same voltage amplitudes at all output levels, can beviewed as a replacement of the single transition point T with anintermediate or transition region limited by two transition points T1,T2.

Referring to FIG. 18, under low output operation, only PA1 suppliescurrent. After PA1 goes into constant voltage operation, PA2 starts todeliver current, while PA1 in this case at first decreases its outputcurrent amplitude in this region. In the low output region there is alsoa phase “offset” compared to the Chireix amplifier.

Unlike the prior art nonlinear Chireix amplifier drive current functionof FIG. 7, the detuned Chireix amplifier has an optimum nonlinear drivecurrent function (in unit 38) that has a phase that varies withamplitude. This can be seen in FIG. 21 as a non-zero imaginary part. Thetwo transitions with an intermediate region, instead of the singletransition point for a tuned Chireix amplifier, can also be seen(indicated by the dashed lines) in this function.

The resulting efficiency curve has a shape that differs from that of thetuned Chireix amplifier, as can be seen in FIG. 22. The transitionregion widens (and lowers slightly) the lowest efficiency peak. Theaverage efficiency can thus be higher, for certain common signalamplitude distributions, for a detuned Chireix amplifier than for anoptimally designed modified (i.e. [13]) Chireix amplifier. The optimalamount of detuning (and of course the general dimensioning) of theamplifier depends on the input signal amplitude distribution at hand.

EXAMPLE 2 Detuned Doherty Amplifier

A Doherty amplifier can be built with transmission lines of λ/4 and λ/2electrical length from the constituent amplifiers to the common output[16]. In the case presented here, such a Doherty amplifier is detuned to19% below its nominal frequency. The lengths of the transmission linesare then instead 0.21λ (from PA1 to the common output) and 0.42λ (fromPA2) which sums to 0.63λ. Such a detuned Doherty amplifier isillustrated in FIG. 23. The amplitudes and phases of the output nodevoltages and output currents for this amplifier are shown in FIG. 24-27.

Comparing with the Doherty amplifier of FIG. 9, the main characteristicof the new solution is shown in the voltage diagram of FIG. 24 as anintermediate or transition region in the upper end where both amplifiersare at constant voltage. As in the detuned Chireix case, this can beseen as the insertion of an extra transition point T2, but this time itis the top (maximum power) point that has been split. Notice that allthe regions of the voltage diagram of the previously studied detunedChireix are there, but with smaller or larger size. The currentsillustrated in FIG. 26-27 are similar to those of the original Dohertyamplifier, but have a region in the upper end where they run morequickly together.

In the top region a difference can be noticed between the presentinvention and regular outphasing, as used in Chireix amplifiers. As inoutphasing, both amplifiers are at maximum voltage, but the phasechanges of the currents or voltages generally are by different amountsat the different output nodes, as opposed to prior art outphasing. Thiseffect is more visible in the detuned Doherty amplifier (see FIG.24-27), than in the detuned Chireix amplifier (see FIG. 17-20).

Unlike the Doherty amplifier of FIG. 9, the detuned Doherty amplifierhas an optimum nonlinear drive current function 22 that has a phase thatvaries with amplitude. This can be seen in FIG. 28 as a non-zeroimaginary part. The two transitions T1, T2, instead of the singletransition T of the Doherty amplifier, can also be seen in thisfunction.

The resulting efficiency curve has a shape that differs from that of theprior art Doherty amplifier mainly by having a Chireix-like uppermostregion. The efficiency in this region is therefore increased compared tothat of the original Doherty amplifier, while the efficiency peak athalf of the maximum amplitude is largely unaffected. This is illustratedin FIG. 29.

EXAMPLE 3 Current Limitation at Low Outputs

This example of a detuned asymmetric (in transistor sizes) Chireixamplifier, illustrates the need for extra transition points, i.e. moreregions with different operating conditions. The transmission lineimpedances are 1.4 (from PA1 to the common output) and 3.5 (from PA2)respectively. This means that the maximum output current from PA1 is1/1.4= 5/7 and from PA2 1/3.5= 2/7. The line lengths, as illustrated inFIG. 30, are 0.33λ (from PA1 to the common output) and 0.19λ (from PA2)which sums to 0.52λ. The amplitudes and phases of the output nodevoltages and output currents for this amplifier are shown in FIG. 31-34.

The point of this example is to show the extra transition (at 0.36 ofthe maximum output voltage) that arises due to the current limitation atzero. As before, one amplifier (PA2) optimally takes care of the firstregion up to T1 by itself (FIG. 33). After this amplifier has reachedits maximum output node voltage, the other amplifier (PA1) mustcontribute in the next region from T1 to T2. The optimal drive in thiscase requires the PA2 output current to decrease during this region, butwhen this current reaches zero, a transition T3 must occur. Thistransition, due to the fact that negative RF currents draw the same DCcurrent as positive ones, is inside the region where one amplifier is atmaximum voltage. The three transitions can also be seen in thedecomposed nonlinear function illustrated in FIG. 35. They can also benoticed in the efficiency curve in FIG. 36.

EXAMPLE 4 Current Limitation at High Outputs

In this example we have an amplifier that can be described as a verydetuned symmetric Chireix amplifier originally designed for a lowtransition point. The example shows the need for extra transition pointsif a current limitation is encountered in optimal operation at highoutput voltages. The line lengths are here, as illustrated in FIG. 37,0.20λ (from PA1 to the common output) and 0.18λ (from PA2) which sums to0.38λ. The amplitudes and phases of the output node voltages and outputcurrents for this amplifier are shown in FIG. 38-41.

The extra transition T3 (at 0.83 of the maximum output voltage) thatarises due to the current limitation of PA1 is clearly seen in both thevoltage plot and the current plot. In the region above this transition,the voltage at PA2 breaks away from its maximum, which was used in theregion below. This can be seen as a splitting of the uppermost region(as described in the first three examples) into two regions. Thedecomposed nonlinear function for this amplifier is shown in FIG. 42.Efficiency-wise, this amplifier is only slightly better than a singleamplifier class B stage, as can be seen in FIG. 43.

Although the embodiments described in examples 3 and 4 are notconsidered to be “good” amplifiers, they illustrate how the presentinvention can be used in special cases.

EXAMPLE 5 Detuned Chireix-Doherty Amplifier

This example shows how to extend the solution to higher-order amplifiersystems [17, 18, 19] by example of a three-transistor amplifier havingChireix properties at low output voltages and Doherty properties at highoutput voltages [18]. The amplifier consists of three equal transistors,coupled with equal impedance transmission lines to a common output. Thetransmission line lengths are: 0.30λ from the PA1 transistor, 0.22λ fromPA2 and 0.60λ from PA3 to the output, as illustrated in FIG. 44. Thefirst two transistors constitute a detuned Chireix pair (4% above theordinary Chireix operating frequency) and the third constitutes the“Doherty part” of the amplifier (detuned to 20% above its ½ frequency).The amplitudes and phases of the output node voltages and outputcurrents for this amplifier are shown in FIG. 45-48. In these plots wecan observe both the behavior of the detuned Chireix, described in thefirst example, and that of the detuned Doherty from the second example.In the highest region, where all amplifiers are at maximum voltage, PA3is “outphased” (in the widest sense) against the two other amplifiers,as seen in the phase plot. The detuned Chireix behavior is seen mainlyin the appearance of a transition region instead of the original singletransition point at 0.25-0.3 of the maximum output voltage.

As seen in FIG. 49, the efficiency curve of this amplifier is higher andmore even than the original [18, FIG. 8]. This is due to the widenedefficiency peaks. The average efficiency of this amplifier can thus bebetter, for many signal amplitude distributions, than the efficiency ofthe “correctly” tuned amplifier.

A common feature of all the described embodiments is that the nonlineardrive function is complex valued (instead of real valued as in the priorart) to compensate for the deficiencies (detuning, output currentlimitations) of the components in order to obtain optimum efficiencyunder the given circumstances. Expressed differently, the presentinvention provides a nonlinear drive function that has a phase thatvaries with the composite amplifier output voltage amplitude. Thisfeature enables the described splitting of transition points intotransition regions, which increases the efficiency.

The nonlinear functions are typically implemented as lookup tables, inwhich each amplitude corresponds to a complex value (amplitude andphase). The complex values can be found experimentally by optimizing theefficiency for each amplitude.

Another way to view the solution besides “transition point splitting” ordrive current modification is to observe the behavior of the voltages atthe output nodes of the transistors. Starting from low output voltages:In the new solution, a single transistor is used for amplification atlow output levels. This transistor provides a linear current withconstant phase offset to the output voltage. If the appropriatetransistor is chosen, this always gives optimal operation in thisregion. When one of the output node voltages reaches its maximum, theother transistor starts operating. The combined operation of the twotransistors in this next region aims to keep one output node at constantvoltage (generally also by having varying phase difference between thetransistors) while using minimum combined drive current magnitudes. Whenthe other transistor's output node voltage also reaches its maximumamplitude, the drive currents are used to keep both transistor outputnodes at constant (maximum) voltage. This is achieved, while stillproviding the required increase in output amplitude, by adjusting thephases of the drive currents along with an increase in amplitude.

Since optimal operation is possible in all of the described amplifiersby single transistor operation at low output levels, efficiency can beincreased by turning off the other amplifier branches, including driveramplifiers, mixers, etc. in this region. This feature can be used bothdynamically, if ramping up/down the hardware can be done with the samespeed as the signal amplitude fluctuations, or quasi-statically, if longperiods of low output levels are encountered. This is possible and canbe used to advantage even in a perfectly balanced Chireix amplifier, byusing the techniques described.

The amount of detuning for optimal efficiency depends, as statedearlier, on the actual signal amplitude distribution. One can howeversay that for Rayleigh-like signal amplitude distributions, slightlydetuned Chireix amplifiers (described in Example 2) are usually theoptimal two-transistor amplifiers. This is because the originalChireix's peak of the efficiency curve is “smeared out” (which in factlowers the peak slightly) over a wider amplitude region. This matchesbetter with the width of the Rayleigh distribution's region of highprobability. In higher-order amplifier systems [17, 18, 19], the rule ofthumb is generally to combine detuned Chireix stages with “smeared”efficiency peaks and detuned Doherty stages with rounded peaks, whileconcentrating the efficiency peaks in the amplitude region(s) of highprobability.

The transitions between regions need not be as sharply defined asdescribed in this document. In fact there can be advantages of having“rounded” transitions, such as reduced bandwidth of the drive signals[20].

In the description above only the most optimal operation for a limitedset of examples has been described. The same ideas can, however, be usedin all amplifiers whose operation is based on output “combination”networks. Even if “optimal” operation is not always achieved, there canbe large gains in using the described techniques.

As indicated by the antennas in the various embodiments, the compositeamplifier in accordance with the present invention may be part of atransmitter, for example a transmitter in a radio terminal, such as abase station or a mobile station in a cellular mobile radiocommunication system.

Some of the advantages of the present invention are:

-   -   General applicability: Can be used in all amplifiers whose        operation is based on output “combination” networks.    -   Counteracts the problems of production variations.    -   Achieves better efficiency with any imperfect amplifier.    -   Obviates trimming.    -   Provides a means for raising the efficiency beyond that of        existing amplifiers, by designing a deliberately detuned        amplifier.    -   More efficient than state of the art amplifiers for any number        of independently driven transistors, for many common signal        amplitude distributions.

As noted above, the methods proposed can be used both for increasingefficiency for imperfect amplifiers, thereby reducing the need fortrimming and/or high-precision manufacturing methods, and for designingamplifiers with optimal efficiency for a given application (amplitudedistribution).

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

REFERENCES

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1. A detuned composite amplifier including a nonlinear drive function,wherein the nonlinear drive function has a phase that varies with thecomposite amplifier output voltage amplitude.
 2. The composite amplifierof claim 1, wherein the composite amplifier is a detuned Dohertyamplifier.
 3. The composite amplifier of claim 2, wherein the nonlineardrive function is configured to produce an operating region with twoamplifiers at maximum output voltage.
 4. The composite amplifier ofclaim 1, wherein the composite amplifier is a detuned Chireix amplifier.5. The composite amplifier of claim 4, wherein the nonlinear drivefunction is configured to produce an operating region with only oneamplifier at maximum output voltage.
 6. The composite amplifier of claim4, wherein the nonlinear drive function is configured to prevent oneamplifier from delivering any output current as long as the outputsignal amplitude of the composite amplifier does not exceed apredetermined threshold.
 7. The composite amplifier of claim 4, whereinthe nonlinear drive function is configured to produce unequal outputcurrent amplitudes from the two amplifiers in at least one outputvoltage region.
 8. The composite amplifier of claim 4, wherein thenonlinear drive function is configured to produce an outphasing regionwith both amplifiers having a constant output voltage but differentphases relative to the composite amplifier output signal phase.
 9. Thecomposite amplifier of claim 1, wherein the composite amplifier is adetuned combined Chireix-Doherty amplifier.
 10. The composite amplifierof claim 9, wherein the nonlinear drive function is configured toproduce an outphasing region with at least two amplifiers having aconstant output voltage but different phases relative to the compositeamplifier output signal phase.
 11. The composite amplifier of claim 1,wherein the nonlinear drive function is configured to drive twoamplifiers in a region with one amplifier at maximum voltage and in aregion with both amplifiers at maximum voltage, the second region beingmore extended than a singular point at max power.
 12. The compositeamplifier of claim 1, wherein the nonlinear drive function is configuredto drive amplifiers in a region with one amplifier at maximum voltage,in at leas one further region with at least one further amplifier atmaximum voltage, and in a region with all amplifiers at maximum voltage,the last mentioned region being more extended than a singular point atmax power.
 13. The composite amplifier of claim 1, wherein the nonlineardrive function is configured to transform at least one output voltagetransition point into an extended output voltage transition region. 14.A transmitter including a composite amplifier in accordance withclaim
 1. 15. A method of driving a detuned composite amplifier includinga nonlinear drive function, said method comprising the step ofconfiguring the nonlinear drive function with a phase that varies withthe composite amplifier output voltage amplitude.
 16. The method ofclaim 15, wherein the composite amplifier is a Doherty amplifier. 17.The method of claim 16, wherein the nonlinear drive function isconfigured to produce an operating region with two amplifiers at maximumoutput voltage.
 18. The method of claim 15, wherein the compositeamplifier is a Chireix amplifier.
 19. The method of claim 18, whereinthe nonlinear drive function is configured to produce an operatingregion with only one amplifier at maximum output voltage.
 20. The methodof claim 19, wherein the nonlinear drive function is configured toprevent one amplifier from delivering any output current as long as theoutput signal amplitude of the composite amplifier does not exceed apredetermined threshold.
 21. The method of claim 19, wherein thenonlinear drive function is configured to produce unequal output currentamplitudes from the two amplifiers in at least one output voltageregion.
 22. The method of claim 19, wherein the nonlinear drive functionis configured to produce an outphasing region with both amplifiershaving a constant output voltage but different phases relative to thecomposite amplifier output signal phase.
 23. The method of claim 15,wherein the composite amplifier is a combined Doherty-Chireix amplifier.24. The method of claim 23, wherein the nonlinear drive function isconfigured to produce an outphasing region with at least two amplifiershaving a constant output voltage but different phases relative to thecomposite amplifier output signal phase.
 25. The method of claim 15,wherein the nonlinear drive function is configured to drive twoamplifiers in a region with one amplifier at maximum voltage and in aregion with both amplifiers at maximum voltage, the second region beingmore extended than a singular point at max power.
 26. The method ofclaim 15, wherein the nonlinear drive function is configured to driveamplifiers in a region with one amplifier at maximum voltage, in atleast one further region with at least one further amplifier at maximumvoltage, and in a region with all amplifiers at maximum voltage, thelast mentioned region being more extended than a singular point at maxpower.
 27. The method of claim 15, wherein the nonlinear drive functionis configured to transform at least one output voltage transition pointinto an extended output voltage transition region.